1. Field of the Invention
The present invention relates to an apparatus for driving a discharge display panel, and more particularly, to an apparatus for driving a discharge display panel using dual subfield coding.
2. Discussion of the Background
Generally, a plasma display panel (PDP) displays images by gas discharge. FIG. 1 is an internal perspective view showing a structure of a conventional three-electrode surface discharge PDP.
Referring to FIG. 1, a conventional surface discharge PDP 1 may include address electrode lines AR1, AG1, . . . , AGm, ABm, dielectric layers 11 and 15, Y electrode lines Y1, . . . , Yn, X electrode lines X1, . . . , Xn, a fluorescent layer 16, barrier ribs 17, and a magnesium oxide MgO layer 12 forming a protective film between upper and lower glass substrates 10 and 13.
The address electrode lines AR1, AG1, . . . , AGm, ABm are formed on the lower glass substrate 13 in a predetermined pattern, and a lower dielectric layer 15 covers the address electrode lines AR1, AG1, . . . , AGm, ABm. The barrier ribs 17 may be formed on the lower dielectric layer 15 in parallel to the address electrode lines AR1, AG1, . . . , AGm, ABm. The barrier ribs 17 partition a discharge space 14 to define discharge cells and prevent optical cross talk between adjacent discharge cells. The fluorescent layer 16 is formed on the lower dielectric layer 15 and on sides of the barrier ribs 17.
The X electrode lines X1, . . . , Xn and the Y electrode lines Y1, . . . , Yn are formed in pairs on a surface of the upper glass substrate 10 facing the lower glass substrate 13, and they extend in a direction substantially perpendicular to the address electrode lines AR1, AG1, . . . , AGm, ABm. Each intersection of an address electrode with an X and Y electrode pair corresponds to a discharge cell. The X electrode lines X1, . . . , Xn and the Y electrode lines Y1, . . . , Yn may comprise transparent electrode lines made of a transparent, conductive material, such as indium tin oxide (ITO), and metal electrode lines for increasing conductivity of the transparent lines. The upper dielectric layer 11 covers the X electrode lines X1, . . . , Xn and the Y electrode lines Y1, . . . , Yn. The protective layer 12, which protects the PDP 1 from a strong electric field, covers the upper dielectric layer 11. A plasma forming gas is sealed in the discharge space 14.
U.S. Pat. No. 5,541,618 discloses a method of driving a PDP such as the PDP 1 of FIG. 1.
FIG. 2 is a timing graph showing a conventional driving method for the PDP of FIG. 1.
Referring to FIG. 2, a unit frame may be divided into 8 subfields SF1, . . . , SF8 in order to realize time division gradient display. The subfields SF1, . . . , SF8 may be further divided into reset periods R1, . . . , R8, addressing periods A1, . . . , A8, and sustain discharge periods S1, . . . , S8.
The PDP's brightness is directly proportional to the length of the sustain discharge periods S1, . . . , S8 in the unit frame. In FIG. 2, the length of the sustain discharge periods S1, . . . , S8 per unit frame is 255T (T is a unit time), and a sustain discharge period Sn of an nth subfield SFn is set to a time corresponding to 2n−1. Accordingly, a total 256 gradients, including gradient 0, may be performed by properly selecting subfields to be displayed among the 8 subfields.
FIG. 3 shows driving signals that may be applied to electrode lines of the PDP 1 of FIG. 1 in a unit subfield of FIG. 2.
Referring to FIG. 3, SAR1 . . . ABm indicates driving signals applied to the address electrode lines AR1, AG1, . . . , AGm, ABm, SX1 . . . Xn indicates driving signals applied to the X electrode lines X1, . . . , Xn, and SY1 . . . SYn indicates driving signals applied to the Y electrode lines Y1, . . . , Yn.
During a reset period PR of a unit subfield SF, a voltage supplied to the X electrode lines X1, . . . , Xn may increase from a ground voltage VG to a first voltage Ve, e.g., to 155V. At this time, the Y electrode lines Y1, . . . , Yn and the address electrode lines AR1, AG1, . . . , AGm, ABm may be biased at the ground voltage VG.
Next, a voltage supplied to the Y electrode lines Y1, . . . , Yn may increase from a second voltage VS, e.g., 155V, to a voltage VSET+VS, e.g., to 355V, which is obtained by adding the second voltage VS to a third voltage VSET. At this time, the X electrode lines X1, . . . , Xn and the address electrode lines AR1, AG1, . . . , AGm, ABm may be biased at the ground voltage VG.
Then, while biasing the X electrode lines X1, . . . , Xn at the first voltage Ve and the address electrode lines AR1, AG1, . . . , AGm, ABm at the ground voltage VG, the voltage supplied to the Y electrode lines Y1, . . . , Yn may decrease from the second voltage VS to the ground voltage VG.
Accordingly, during a subsequent address period PA, addressing can be smoothly performed by applying display data signals to the address electrode lines AR1, AG1, . . . , AGm, ABm and sequentially applying scanning signals of the ground voltage VG to the Y electrode lines Y1, . . . , Yn, which are biased to a fourth voltage VSCAN that is less than the second voltage VS. A positive polarity address voltage VA is supplied to an address electrode line AR1, AG1, . . . , AGm, ABm to select a discharge cell, and the ground voltage VG is supplied to an address electrode for a discharge cell that is not to be selected. Accordingly, simultaneously applying the address voltage VA to one of the address electrode lines AR1, AG1, . . . , AGm, ABm and the scanning signal of the ground voltage VG to one of the Y electrode lines Y1, . . . , Yn generates an address discharge in the corresponding discharge cell, thereby forming wall charges in the cell. At this time, the X electrode lines X1, . . . , Xn may be biased at the first voltage Ve for a more reliable addressing operation.
During a subsequent sustain discharge period PS, alternately applying the a sustain discharge pulse of the second voltage VS to the Y electrode lines Y1, . . . , Yn and the X electrode lines X1, . . . , Xn generates a sustain discharge in selected cells, thereby displaying an image.
FIG. 4 is a graph showing degrees of freedom of gradients with respect to gradient levels when the gradients are expressed by dividing each frame into 10 subfields. FIG. 5 is a table showing subfield coding results with respect to gradient levels when the gradients are expressed by dividing each frame into 10 subfields.
Referring to FIG. 4 and FIG. 5, 256 gradients are expressed by dividing each frame into 10 subfields, and degrees of freedom of gradients and subfield coding results when the 10 subfields having gradient weights of 1, 2, 4, 8, 16, 25, 35, 45, 55, and 64 are shown. Here, each subfield code word of FIG. 5 is performed in the order of SF1, SF2, . . . , SF10. Since each subfield may have a relevant gradient redundancy at each gradient level by expressing the gradients as shown in FIG. 4, generation of a problem can be prevented by substituting a subfield set having a possibility of generating a problem with another subfield set expressing the same gradient.
When gradients are expressed by dividing each frame into 8 subfields as shown in FIG. 2, 28=256 gradients may be expressed. At this time, gradient weights of the 8 subfields are expressed as 2n−1, i.e., 1, 2, 4, 8, 16, 32, 64, and 128, and there is no gradient redundancy. However, in this case, pseudo-contours generated by changing a subfield set displayed whenever a gradient increases due to an increase of a subfield representing a moving picture cannot be prevented. In this case, this pseudo-contour problem can be solved by expressing gradients using a subfield set with which the problem is not generated by increasing the number of subfields configuring each frame while expressing the same gradient.
A PDP writes data on subfields to be displayed through the address discharge, which is generated by applying data pulses and scan pulses to address electrodes and scanning electrodes, respectively. Since a discharge delay time is necessary to generate the address discharge, the discharge delay time determines the length of an address period.
This address discharge delay time is largely affected by priming due to the address discharge of adjacent cells. That is, when adjacent cells are addressed, the address discharge delay time decreases, resulting in a high probability of a successful address discharge. On the contrary, when adjacent cells are not addressed, the probability of a successful address discharge decreases. Since the probability of successful address discharge may be very low when many addressed cells are not adjacent to other addressed cells, a failure of the address discharge may result in a failure of the sustain discharge, which may result in poor gradient expression. In particular, when the address discharge failure occurs in a subfield having a large gradient weight, since a low gradient discharge effect that a high gradient is intermittently not expressed may occur very severely, the probability of success of the address discharge should be very high in subfields having a large gradient weight.
In a conventional PDP, a value of an input gradient is converted from an integer to a rational number through a gamma block in order to express a low gradient, and an error diffusion block may convert an error of gradient data into the rational number. For example, when a value of a gradient input from the gamma block is a rational number equal to 56.0625, the gradient equal to 56.0625 can be expressed by combining a gradient equal to 56 and a gradient equal to 57 in a proper ratio in order to express 56.0625 using the error diffusion block. When using the subfield coding shown in FIG. 5, subfield code words corresponding to 56 and 57 are ‘1111110000’ and ‘0110101000’, respectively.
When 56.0625 is expressed by a spatial combination of 56 and 57, data switching occurs in SF1, SF4, SF6, and SF7. Since the value is 56.0625, the gradient equal to 56 may be turned on in a distribution ratio of about 93.8% of a predetermined area, and the gradient of 57 may be turned on in a distribution ratio of about 6.2% of the predetermined area. Here, a probability of success of the address discharge of SF7 of the gradient equal to 57 may cause a problem. That is, since SF7 equal to 57 is not turned on in a previous subfield, a priming effect by a sustain discharge of the previous subfield does not exist, and since most of adjacent cells are gradients equal to 56, SF7s of the adjacent cells do not have address data. Accordingly, a priming effect by addressing the adjacent cells does not exist. Therefore, an address discharge may be performed under conditions of a very scarce priming effect due to a solo addressing, and this may cause the low gradient discharge effect.
If the subfield codeword equal to 56 is made to be similar to the subfield codeword equal to 57 within a range permitted by a relevant gradient redundancy, a low discharge in a low gradient may be reduced. However, in this case, since the gradient low discharge between 56 and 57 is moved to a gradient low discharge between 55 and 56, this does not mean that a gradient in which the gradient low discharge is generated disappears, rather it means it may transition to another gradient.
That is, when the gradient switching is performed in a subfield having a large gradient weight, the low discharge can be generated in a low gradient, and this may cause a very poor gradient expression of a PDP. In particular, when an input gradient passes through the gamma block, most gradients move to a low gradient region. For example, when an input gradient is 100, a gradient level may decrease to about 20 at a back end of the gamma block. In this case, most gradients may be expressed with subfields having low gradient weights, and when subfields are designed using the subfield weights shown in FIG. 5, since a least significant bit (LSB) subfield does not have a redundancy, the solo addressing by the subfield switching occurs due to the error diffusion. Accordingly, a low discharge effect in a low gradient may be severe.
That is, the error diffusion satisfies g<x<g+1 by spatially combining a gradient g+1 with respect to a gradient g in order to express a gradient x including a value below a decimal point. At this time, since the subfield coding of the gradient g and the gradient g+1 according to the gradient x may vary largely, the low discharge effect in rapidly varied subfields may be severe.